Lost foam and permanent mold casting processes are often utilized to cast complex metal articles, such as engine blocks. It is well documented that the lost foam casting process is an efficient and effective casting process for forming such articles. See U.S. Pat. Nos. 4,854,368; 5,014,764; 5,058,653; 5,088,544; 5,161,595; and 5,960,851. Likewise, permanent mold casting is an effective means for the production of complex metal articles, and is well known in the art of molten metal casting.
One of the advantages of the lost foam casting process is that it is capable of forming complex internal passageways during casting, such as the complex internal passageways of an internal combustion engine. In lost foam casting, a pattern is produced from a polymeric foam material, such as polystyrene, and has a configuration identical to the metal article to be cast. A porous ceramic coating is subsequently applied to the outer surface of the pattern and one or more patterns are placed within an outer vessel. A polymeric foam gating system connects each pattern to a sprue in order to supply the molten metal to the pattern. The space between the patterns and the vessel is filled with a finely divided inert material, such as sand, and the finely divided inert material also fills the internal cavities within the pattern.
In the lost foam casting process, as molten metal is fed into the sprue, the heat of the molten metal acts to decompose or ablate the polymeric foam material comprising the pattern and the gating system. The molten metal occupies the void created by ablation of the foam material, with the decomposition products of the foam passing through the porous ceramic coating of the pattern and becoming trapped within the interstices of the sand. Upon solidification of the molten metal, the resulting cast article has a configuration identical to the original polymeric foam pattern. However, due to the decomposition products becoming trapped within the interstices of the sand, a lost foam cluster or bonded cluster surrounds the cast article after solidification. While there is marginal difficulty in removal of the lost foam cluster surrounding the cast object, there is significant difficulty in removal of the residual ceramic coating from the complex internal passageways of the casting.
For this reason, permanent mold casting is often used instead of lost foam casting for the production of complex cast articles. Permanent mold casting allows for articles to be cast in “dies” that are used time and again for casting articles. While permanent mold casting does not necessitate the intensive clean-up associated with the lost foam casting process, problems arise with cast articles “sticking” to the dies. Further, limits on the complexity of the article to be cast exist, and complex articles are often cast in separate sections using permanent mold casting, which requires later assembly of the sections and may cause variation in the metallographic structure of the separate sections.
A significant thrust of the complex casting industry is directed to the production of engine blocks or engine block heads, particularly, with the advent of aluminum alloy engine blocks having high tensile and yield strengths along with desirable elongation percentages (i.e., heightened ductility). In order to achieve such desirable characteristics in the final cast article, precipitation strengthening of aluminum alloys is performed on the cast articles.
Precipitation strengthening of an aluminum alloy is generally accomplished by a three step process: solution heat treatment, quenching and aging. With most cast articles it is desirable to solution heat treat the articles after they are cast. In general, solution heat treating is the process by which an alloy is elevated to a high temperature, thereby changing its microstructure to improve its properties. Though this thermal treatment, the resulting properties and performance of a component may be manipulated. Specifically, when dealing with aluminum silicon alloys, solution heat treatment changes the alloy's microstructure by spherodizing and coarsening eutectic silicon particles, and homogeneously redistributing precipitate forming elements in solid solution. It is known in the art that the heat-up rate and the time spent at solution heat treatment temperature are important factors in obtaining the properties which will increase performance of a heat treated article.
Quenching refers to the rapid cooling of a cast object. Quenching is traditionally done in water. However, new quenching techniques have been developed where other types of fluids are used for quenching. Quenching momentarily “freezes” the eutectic structure, and renders the alloy workable for a short period of time.
The aging process generally follows quenching to allow for slow precipitation of alloy constituents to create a stronger final structure. In traditional, natural aging, a cast object is held at a low temperature (e.g., room temperature) for an extended period of time to allow for precipitation of constituents. When dealing with aluminum silicon alloys, it is known in the art to place cast objects into an air furnace at a relatively higher temperature (e.g., 250 to 450° F.) for a relatively long period of time (e.g., 4 to 72 hours) after quenching. This traditional aluminum silicon alloy aging process is called artificial aging, and this very practical and popular aging process also allows for the formation of fine strengthening precipitates which creates a stronger final cast article.
The traditional aging process for aluminum silicon alloys, described above, follows thermodynamics and Ahrenious kinetics. Such principles teach that the maximum strength of an alloy is obtained by aging at a lower temperature (thermodynamic consideration) for longer times (kinetics consideration—i.e., slower reaction rates at low temperatures). In a commercial setting, however, this traditional aging method causes a large capital cost and productivity hindrance, as it ties up a substantial amount of furnace capacity during production. Additionally, this traditional aging method uses more energy and would be desirable in order to obtain statistically guaranteed levels of strength in manufactured aluminum alloy parts.
Many attempts have been made in the aluminum industry to create alloys with fine strengthening participates using modified aging processes. Metallurgists have attempted to raise the temperature and/or shorten the time for aging, but such attempts have resulted in a large variability in levels of strength from production lot to production lot. Therefore, the traditional aging methods described above remain commonplace in the industry.
In accordance with the present invention, the use of a fluidized bed has been found to advantageously and economically solution heat treat and age cast aluminum alloy articles. The economy and efficiency of the present invention is achieved through the use of a heated, fluidized bed of an inert material, such as sand, because such beds allow for excellent temperature control which, in turn, provides excellent temperature stability at elevated temperatures. The fluidized beds contemplated for use in conjunction with the present invention is described in U.S. Pat. No. 6,042,369 which is incorporated herein by reference. The fluidized aging bed, as well as the fluidized bed used for quenching, has the same construction as the heat treatment bed 4 and, preferably, is the heated fluidized sand bed described in U.S. Pat. No. 6,042,369. The fluidized beds, as described therein, are very accurate and deviate very little from the desired heat treatment or aging temperature. Thus, one may obtain statistically guaranteed strength in cast articles by heat treating and aging such articles at an elevated, stable temperature for a shorter period of time when compared to conventional solution heat treating and aging processes.
Accordingly, in the aluminum alloy casting industry, the method of the present invention allows one to realize significant production efficiencies while maintaining very high product quality. Specifically, an article may be cast in the morning, solution heat treated around noon, and machined in the afternoon. It is estimated that a minimum of 2 to 4 days of work in process can be eliminated, while quality is improved because the method of the present invention is a continuous process that yields higher statistically guaranteed properties due to excellent temperature control. Thus, the net result is a significant amount of energy savings in conjunction with improved product quality.
A further advantage of the method of the present invention is realized because it has been found that the fluidized action of the beds efficiently and effectively cleans residual ceramic coatings from complex cast articles to a degree that cannot be realized with prior cleaning methods. Even further, it has been found that the lost foam cluster can be directly transferred from the lost foam casting vessel to the fluidized bed to allow for greater economy in the overall casting process.
Therefore, the present invention provides generally for a method of manufacturing a complex aluminum alloy article in a refined time period and further comprises a continuous manufacturing process for engine blocks. The process generally comprises casting a complex aluminum alloy article, solution heat treating the complex article in the first fluidized bed, quenching the complex article, aging the complex article in a second fluidized bed and machining the complex article.
More specifically, the method of the present invention provides for continuous manufacturing of engine blocks and/or engine block heads using the lost foam casting process, wherein the bonded clusters that surround the cast articles resulting from the lost foam casting process are transferred directly into a first fluidized bed. The cast engine blocks and/or engine block heads are solution heat treated in the first fluidized sand bed while, simultaneously, the bonded clusters are removed from around the engine blocks and/or heads and the internal passageways of the engine blocks and/or heads are cleaned. The engine blocks and/or heads are then removed from the first fluidized bed and quenched, preferably in a separate fluidized bed. The quenched engine blocks and/or heads are then transferred to yet another fluidized bed where the engine blocks and/or heads are aged at a desired aging temperature, preferably about 385° F. The engine blocks and/or heads are removed from the aging fluidized bed after a time period from 30 to 60 minutes and are subsequently machined to form a finished product.
One of ordinary skill in the art will realize that this streamlined process provides a multitude of efficiencies in the manufacturing process of complex aluminum alloy articles. Most importantly, production efficiencies are realized through time savings, as well as energy savings, resulting in a more lean manufacturing environment.